Simple Component and Continuity Tester
This simple project may be used for testing components, as well as checking circuit board tracks, wires
and connections for continuity (conduction). It tries to pass a small current through the item being tested
and the LED will light brightly, dimly or not at all according to the resistance of the item:
• LED bright means the resistance is low, less than about 1kΩ
• LED dim means the resistance is medium, a few kΩ
• LED off means the resistance is high, more than about 10kΩ
When not in use the 9V PP3 battery should be unclipped or the crocodile clips attached to a piece of card
or plastic to prevent them touching. You could add an on-off switch in the red wire from the battery clip
and this may be the best option if you mount the simple tester in a box.
Parts Required
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resistor: 390Ω
red LED 5mm diameter, standard type
battery snap for 9V PP3
crocodile clips: miniature red and black
stripboard: 5 rows × 7 holes
Stripboard Layout
Red LED (long lead is +)
9V PP3
battery clip
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390
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red
Miniature crocodile clips
black
Testing stripboard, PCB tracks, wires and connections
Connect a crocodile clip on each side of the suspected fault:
• LED bright means there is a connection.
• LED off means there is no connection.
If you are testing a stripboard or PCB which has components
soldered in place, beware of possible connections via the
components and allow for this when interpreting the results.
390
9V
Stripboard circuits can suffer from two common problems:
solder bridging between adjacent tracks making a connection
where there should be none, and tracks broken with a track
cutter which have an almost invisible thread of copper
conducting across the break.
red LED
+
wire or part
to be tested
If a PCB has etched poorly the tracks may be very thin in places or there may be traces of copper
bridging between adjacent tracks.
Wires and connections may be checked for continuity (conduction).
Please see the next page for information about testing components...
THE
ELECTRONICS
CLUB
© John Hewes 2006, The Electronics Club,
www.kpsec.freeuk.com
A kit for this project is available from
RSH Electronics
Testing components
Connect a crocodile clip on each side of the component. They
can be connected either way round unless stated otherwise in
the table below.
Component
Test results for a component in good condition
LED bright for low resistance, less than about 1kΩ.
LED dim for medium resistance, a few kΩ.
LED off for high resistance, more than about 10kΩ.
Resistor
Across the two ends of the track the LED brightness will depend on the resistance
value (see above).
Between one end of the track and the wiper you should see the LED brightness vary
as you adjust the variable resistor. However, for high resistances (>10kΩ) the LED will
only light near one end of the track.
Variable Resistor
LED bright with red lead to anode and black lead to cathode (stripe).
LED off with black lead to anode and red lead to cathode (stripe).
Diode
a
k
a = anode, k = cathode (the end with a stripe)
LED bright with red lead to anode and black lead to cathode (stripe).
LED dim with black lead to anode and red lead to cathode (stripe) if the zener diode
voltage is less than about 7V.
LED off with black lead to anode and red lead to cathode (stripe) if the zener diode
voltage is greater than about 7V.
a = anode, k = cathode (the end with a stripe)
LED bright with red lead to anode and black lead to cathode (short lead) - the LED
being tested will also light.
LED off with black lead to anode and red lead to cathode (short lead).
Zener Diode
a
k
LED
a
k
flat
Transistor
C
B
C
B
E
E
NPN
PNP
B = base, C = collector,
E = emitter
Please refer to a book or
supplier's catalogue to
identify the leads.
Capacitor
less than 1µF
Capacitor
1µF and greater
LDR
Light Dependent Resistor
Thermistor
Lamp
Switch
Fuse, Motor,
Loudspeaker, Inductor,
Relay coil, Wire
THE
ELECTRONICS
CLUB
a = anode (long lead), k = cathode (short lead, flat on body)
For each pair of transistor leads connect the tester leads first one way, then the other
way.
These are the results for an NPN transistor in good condition:
CE pair: LED off both ways.
BC pair: LED bright with red lead on B, LED off the other way.
BE pair: LED bright with red lead on B, LED off the other way.
These are the results for a PNP transistor in good condition:
CE pair: LED off both ways.
BC pair: LED bright with black lead on B, LED off the other way.
BE pair: LED bright with black lead on B, LED off the other way.
Note that you can use the tester to identify the B lead (the one which always conducts
one way) and to distinguish NPN and PNP transistors (by the tester lead colour when B
conducts). However, the tester cannot distinguish the C and E leads.
LED off.
Please bear in mind that a broken connection will give the same result.
If the capacitor is polarised (most will be) connect the red lead to positive (+) and the
black lead to negative (-).
The LED will flash briefly when first connected.
Reverse the connections: the LED will give another brief flash.
With low values like 1µF the flash will be almost too brief to see, but larger values such
as 100µF will give longer flashes. Electrolytic capacitors may leak a little when
connected the wrong way round, making the LED light dimly continuously.
LED bright when the LDR is in bright light.
LED dim when the LDR is in normal room light.
LED off when the LDR is in darkness.
LED dim when the thermistor is warm.
LED off when the thermistor is cold.
These are typical results, exact results depend on the thermistor's resistance.
LED bright.
Note that the lamp itself will NOT light because the test current is too small.
LED bright when switch contacts are closed (on).
LED off when switch contacts are open (off).
Note that you can use the tester to identify the switch contacts if necessary.
LED bright.
© John Hewes 2006, The Electronics Club,
www.kpsec.freeuk.com
A kit for this project is available from
RSH Electronics